WASHINGTON, March 13 (Reuters) - Congress plans to scrutinize why the United States waited so many days to ground all Boeing Co 737 MAX jets involved in Sunday’s crash in Ethiopia as other countries and airlines acted more quickly.

The Federal Aviation Administration said the order on Wednesday was the result of “new evidence collected at the site and analyzed today” and “newly refined satellite data” that Canada had cited earlier in its decision to halt flights.

The FAA did not disclose the new evidence at the scene but said it was “the missing pieces” that aligned the track of the two fatal Boeing 737 MAX 8 crashes since October.

For decades, the United States has led the world in aviation safety, often setting standards that were later adopted by other countries. The agency came under heavy criticism from U.S. lawmakers and others who questioned why the FAA waited so long to ground the Boeing 737 MAX.

FAA officials plan to brief lawmakers Thursday, two people familiar with the matter told Reuters.

While President Donald Trump announced the ban on television, acting FAA Administrator Dan Elwell said he made the decision with the support of Transportation Secretary Elaine Chao.

“We were resolute in our position that we would not take action until we had data to support taking action,” Elwell told reporters. “That data coalesced today and we made the call.”

Canada grounded the planes earlier on Wednesday while the European Union acted on Tuesday. China and some airlines ordered the planes not to fly within hours of the crash on Sunday.

As of Wednesday night, regulators in Argentina and Mexico had not grounded planes.

House Transportation and Infrastructure Committee Chairman Peter DeFazio, a Democrat, said “it has become abundantly clear to us that not only should the 737 MAX be grounded but also that there must be a rigorous investigation into why the aircraft, which has critical safety systems that did not exist on prior models, was certified without requiring additional pilot training.”

Elwell said Wednesday he was confident in the 737’s certification.

The Senate Commerce Committee also plans to hold a hearing as early as April. Senator Ted Cruz said he plans “to investigate these crashes, determine their contributing factors, and ensure that the United States aviation industry remains the safest in the world.”

The grounding was an abrupt reversal as the United States had repeatedly insisted the airplane was safe to fly even as regulators and airlines around the world grounded the airplane.

Trump spoke to Boeing Chief Executive Dennis Muilenburg on Wednesday before the announcement.

United Airlines, American Airlines and Southwest Airlines Co all fly versions of the 737 MAX and immediately halted flights on Wednesday.

American, with 24 737 MAX airplanes, said it will be “working to re-book customers as quickly as possible, and we apologize for any inconvenience.”

Boeing said it supported the action to temporarily ground 737 max operations after it consulted with the FAA, NTSB and its customers. Boeing shares were down 2 percent.

The shift came less than a day after U.S. regulators had again insisted the plane was safe. Even Chao flew aboard a 737 MAX on Tuesday.

The FAA plans to mandate design changes by April that have been in the works for months for the 737 MAX 8 fleet. Boeing said late Monday it will deploy a software upgrade across the 737 MAX 8 fleet “in the coming weeks.”

The company confirmed it had for several months “been developing a flight control software enhancement for the 737 MAX, designed to make an already safe aircraft even safer.”

The FAA said the changes will “provide reduced reliance on procedures associated with required pilot memory items.”

Elwell said Wednesday he was hopeful software improvements “will be ready in a couple months” after testing and evaluation is completed by the FAA of what he called a “software patch.”

Reporting by David Shepardson and Steve Holland; Additional
reporting by Ginger Gibson; Writing by Tim Ahmann; Editing by
Nick Zieminski and Lisa Shumaker